Circuit boards now available and having switches, rheostats, and other electrical parts come in two kinds. As one kind, flexible circuit boards made of polyethylene film or the like and on which patterns of electrical parts such as switches or other circuit patterns have been previously formed, the boards being bonded to retaining plates. Previously fabricated rheostats have protrusions extending from the bottom, the protrusions being fitted in holes that extend through both the flexible circuit board and the retaining plate. Thus, the rheostats are firmly fixed to the flexible circuit and the retaining plate. Resistance patterns and charge-collective patterns are formed on the circuit board. Connection terminals are connected to the resistance patterns and the charge-collecting patterns, and are formed in proximity to the rheostats on the flexible circuit board. The connection terminals and the connection patterns are electrically connected with the edge connector of the circuit board using known electrically conductive heat-seal film. Circuit boards of another kind consists of heat-resistant film made from polyimide or other similar material. Resistors of switches and rheostats, charge collectors, patterns of electrical parts, and other circuit patterns are all formed on the circuit board, which is bonded to a retaining plate. Slider supports to which the sliding contacts of rheostats are rigidly fixed have retaining portions engaged in holes that extend through the circuit board and the retaining plate.
In the first-mentioned conventional circuit board, the connection terminals of the rheostats are not coplanar with the connection patterns of the flexible circuit board. Further, the casings of the rheostats have protrusions. Therefore, when the conductive heat-seal film is heated, it cannot be stably bonded. This can result in poor connection. In the last-mentioned conventional circuit board, switches, rheostats, and other patterns are formed on the same film, so that patterns are reliably connected with each other. However, those patterns are formed in overlapped relation by ordinary printing techniques at several manufacturing steps. Although the area of the resistance patterns accounts for only a small proportion of the area of the circuit board, the number of manufacturing steps per unit time is very small, because the large area of the circuit board must be fabricated during one manufacturing process. Hence, the manufacturing cost cannot be reduced.